As we suspected back in May based on spy shots, Chevrolet is adding a bi-fuel version of the tenth-generation Chevy Impala that can run on both regular gasoline and compressed natural gas (CNG). Unveiled today in Washington, D.C. by General Motors CEO Dan Akerson, the new bi-fuel Impala will be offered to both retail and fleet customers, and Chevy points out that it is the only manufacturer-produced, fullsize bi-fuel sedan in North America. Outside suppliers offer add-on CNG kits, but this new I